A warning, parking in Metro parking lots requires a special card, called a SmartTrip card. You can't pay cash/cc or anything but this card. You can purchase them at any station (I think) and they can also be used to get on and off the trains. It will hold a set value and can be recharged. They are convenient.

You can find out info about the SmartTrip card at http://www.wmata.com/riding/smartrip.cfm.

Also, you can not eat or drink on the trains. This is enforced. :-) Consequently, they tend to be pretty clean, but it can be annoying.